The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to the imaging of moving objects.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.l) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.l is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles, or "views", in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Many magnetic resonance imaging studies require the sequential acquisition of multiple images from the same region of interest in the imaged subject. Pixel-by-pixel data analysis of such images customarily assumes that corresponding pixels in different images represent the same location in the subject. Patient motion is a significant problem and can be a serious source of error in these quantitative procedures. Image registration post-processing schemes can be useful to correct for in-plane motion and navigator echoes can provide additional information regarding translational and/or rotational motion to assist with image registration. These methods are, however, unable to correct for translations and/or rotations of the region of interest out of the imaging volume.
One example of such procedures is intra-operative imaging. Intra-operative MR imaging is employed during a medical procedure to assist the doctor in guiding an instrument. For example, during a needle biopsy the MRI system is operated in a realtime mode in which image frames are produced at a high rate so that the doctor can monitor the location of the needle as it is inserted. A locator device such as that described in U.S. Pat. Nos. 5,622,170 and 5,617,857 may be used to track the location of the instrument and provide coordinate values to the MRI system which enable it to mark the location of the instrument in each reconstructed image. The medical instrument is attached to a handpiece that is manipulated by the physician and whose position is detected by surrounding sensors. For example, the handpiece may emit light from two or more light emitting diodes which is sensed by three stationary cameras.
Tracking devices which employ the MRI system to locate markers in the patient have also been developed. As described in U.S. Pat. No. 5,715,822, such tracking systems employ a small coil attached to a catheter or other medical device to be tracked. An MRI pulse sequence is performed using the tracking coil to produce transverse magnetization at the location of the tracked device. The location of the tracking coil is determined and is superimposed at the corresponding location in a medical image acquired with the same MRI system